Definition is a basic activity of language, of particular importance to linguists because of its use of language to describe itself. Beyond this inherent significance as a crucial element of language study, definitions also provide a rich potential source of the information needed for Natural Language Processing systems. This book describes an investigation of the subset of general language used in definition sentences and the development of a taxonomy of definition types, a grammar of definition sentences and parsing software which can extract their functional components. The work is based on definition sentences used in one of the dictionaries from the Cobuild range, and the book includes a brief history of the development of monolingual English dictionaries, an assessment of the concepts of sublanguages and local grammars and a full exploration of the results of the analysis and of the present and future applications of the taxonomy, grammar and parser.